Process and apparatus for oxidizing hydrocarbons



April 26, 1966 N. R. BEYRARD PROCESS AND APPARATUS FOR OXIDIZINGHYDROCARBONS Filed May 12, 1961 4 Sheets-Sheet 1 W 554M414, {7 MW 6 Lam/S dys April 26, 1966 N. R. BEYRARD PROCESS AND APPARATUS FOR OXIDIZINGHYDROCARBONS Filed May 12, 1961 4 Sheets-Sheet 2 April 26, 1966 BEYRARD3,248,453

PROCESS AND APPARATUS FOR OXIDIZING HYDROCARBONS Filed May 12, 1961 4Sheets-Sheet 3 PROCESS ANb APPARATUS FOR OXIDIZING HYDROCARBONS 4Sheets-Sheet 4 Filed May 12,

MM 022$ E 5874:1 24

0 MW 2 w/es, 404 .r.

United States Patent 3,248,453 PROCESS AND APPARATUS FOR OXIDIZINGHYDROCARBONS Norbert R. Beyrard, Paris, France, assignor to Societe deSynthese et dOxydation Synoxy, Paris, France, a company of France FiledMay 12, 1961, Ser. No. 109,633 Claims priority, application France, May18, 1960, 827,598 9 Claims. (Cl. 260687) This invention relates toprocesses and apparatus for oxidising hydrocarbons.

It is known that it is possible to obtain valuable synthetic products bycontrolled oxidation of hydrocarbons in the presence of catalysts. Forexample, phthalic anhydride can be obtained by oxidation of naphthaleneor o-xylene by means of catalysts such as vanadium oxides. Generallyspeaking, the oxidising substance is simply atmospheric air in which thehydrocarbon to be oxidised is incorporated in the form of vapour in aquantity below the explosion threshold.

It is also known that the temperature at which a particular product isobtained is somewhat critical. If this temperature is not reached, theoxidation is not obtained, while if it is exceeded the oxidisingreaction is accelerated and the hydrocarbon tends to be completelydestroyed.

Since the oxidising reaction is exothermic, it has already beenproposed, in order to maintain the reaction temperature at the desiredvalue, to incorporate in the catalysing chamber heat exchangers whoseoperating temperature is stabilised by a thermostat.

It has also been proposed to divide the reaction into a succession ofstages each effected in a different catalysing chamber and, whilecirculating the reaction mixture from one chamber to the other, to coolthis mixture as it leaves one chamber and before it enters thesucceeding chamber.

This cooling can be effected by means of a heat exchanger, or byinjection of a cold fluid diluent into the mixture. In particular, ithas been proposed to inject a predetermined quantity of liquid waterbetween two successive chambers, the vaporisation of this watereffecting the desired cooling.

Although the dilution of the reaction mixture by means of water scarcelyhas any disadvantages effect on the reaction itself, the thermal energydissipated by the evaporation of this water is difficult to recoversince, finally, the extraction of the product obtained is effected bycondensation at low temperature. The water injected into the mixturethen condenses at this low temperature and interferes with the saidcondensation without having any useful effect.

The use of intermediate coolants between two successive reactionchambers complicates the construction of the installations and increasestheir cost when it is desired to recover the heat absorbed by thesecoolants.

It is an object of the present invention to provide an oxidising processwhich obviates these disadvantages, and apparatus for carrying out suchprocess.

According to the present invention there is provided a process for themultistage exothermic oxidation of hydrocarbons by means of atmosphericair, which comprises bringing the reaction mixture of hydrocarbon vapourand oxidising gas in successive adiabatic stages into contact withsuccessive quantities of catalyst at a temperature below the optimumreaction temperature, and cooling the mixture, after the said contact,from the temperature, above the said optimum temperature, which itreaches, to the said lower temperature, the said cooling being effectedby the addition of an appropriate quantity of air and the final mixture,obtained after the last stage, being directed to at least one heatexchanger for recovering the heat content of the said mixture.

Thus, a single heat exchanger may be sufficient to recover, at thetemperature level at which the mixture is discharged, all the utilisablecalories contained therein.

Since the quantity of heat thus recovered is largely in excess of thatrequired for the heating of the mixture entering the installation, in anadvantageous embodiment of the invention the flow leaving the last stageis divided into two portions entering two separate heat exchangers, oneof which is intended for heating the initial mixture of air andhydrocarbons entering the installation, while the other supplies usefulheat for the accessory services of this installation, for example forpurifying by distillation the product obtained.

As will hereinafter be shown, it is particularly advantageous for thequantities of air injected between two successive stages of the reactionto increase with a-geometrical progression, the ratio of which is lowwhen the fraction of hydrocarbon converted at each of the stages issmall.

The apparatus for carrying out the process of the invention may be verysimple, since it is essentially sufficient, between two successivestages, to ensure an admission of air into the mixture and to effecthomogenisation of this air and the mixture before it comes into contactwith the catalyst of the succeeding stage.

Consequently, in its preferred and simplest construcitional form, thisapparatus comprises a vertical tower in which the reacting mixturecirculates from the top downwards, which tower comprises a succession ofpermeable stages supporting beds of catalyst and, between two successivestages a lateral air admission duct opening immediately below one stage,and below this duct a diffusion grid which ensures homogenisation of thecooling air and of the mixture before they come into contact with thestage situated immediately below.

Instead of introducing pure air for cooling the reaction, at each of thestages, it may be more advantageous to introduce air containing anaddition of fresh hydrocarbon already homogenised in the said additionalair.

It is also possible to increase the yield of the successive catalystbeds and the final quantity of product obtained.

The injection of fresh hydrocarbon between two stages of the reaction isknown per se. However, the direct injection of fresh hydrocarbon intothe reaction mixture at high temperature, as already proposed, hasserious disadvantages, because owing to the temperature of the mixture,it is difficult with such an injection to avoid a local excessiveconcentration of hydrocarbon exceeding the ignition threshold of thelatter. The risks of ignition and even explosion of the mixture aretherefore very serious.

On the other hand, if this fresh hydrocarbon is already diluted andhomogenised with the additional cooling air when supplied, these dangersdisappear and the operation can be carried out safely and with fulladvantage.

The quantities of cooling air and of fresh hydrocarbon may bepre-adjusted. However, experience shows that the catalytic actions arenot absolutely constant in time and that an additional adjustment mayprove necessary. This adjustment has essentially the object ofmaintaining on entry into contact with a bed of catalyst and on leavingthe latter, two clearly defined temperature levels on either side of themost favourable temperature level for the desired reaction.

In accordance with one feature of the invention, the adjustment of thequantity of cooling air is rendered dependent upon the temperature ofthe mixture before it comes into contact with a catalysing bed, whilethe adjustment of the quantity of fresh hydrocarbon added to this air isrendered dependent upon the temperature at which it leaves the saidcatalysing bed.

carbon is sent into the mixture, which tends to raise the said outlettemperature.

According to a further feature of this invention there is providedapparatus suitable for carrying out the aforesaid processes whichcomprises a vertical tower and means for feeding a reaction mixturethrough the tower from the top downwards, the said tower comprising aplurality of superimposed units each comprising a stage to carrycatalyst, a gas-diffusion grid located above the stage and spacedtherefrom, and a lateral duct for the supply of air opening immediatelybelow the stage of one unit and above the gas-diffusion grid of the nextlower unit.

A form of apparatus according to the present invention is illustrated inthe accompanying drawings in which:

FIGURE 1 is an overall diagrammatic view of an installation according tothe invention.

FIGURE 2 is a fragmentary section of the reaction tower in the simplestconstructional form.

FIGURE 3 illustrates a variant of the overall layout of theinstallation.

FIGURE 4 is a fragmentary section of the tower illustrated in FIGURE 3.

FIGURE 5 is a diagrammatic illustration of a reaction tower with whichvarious rates of operation are possible.

In the installation illustrated in FIGURE 1, the atmospheric airentering through the filter 1 is sent by the compressor 2 into the duct3, which divides it into two flows by means of the diffuser-distributor4.

The flow passing through the duct 5 is sent into a heater 6 havingauxiliary heating, for example of electrical form, and then into aheating heat exchanger 7. It then enters the chamber 8 in whichhydrocarbon under pressure, supplied at evaporation temperature, isdiffused through the nozzle 9. The diffusion grids 10 effecthomogenisation of the mixture of air and hydrocarbon.

' On leaving the chamber 8, the mixture passes through a heater 11 andthen through the heating heat exchanger 12.

From there, the mixture passes through the duct 13 to the top of thereaction tower 14 which is divided into stages.

' The second flow issuing from the diifuser-distributor 4 first passesthrough a heat exchanger 15 and is then fed through a duct 16 which isvertical and parallel to the tower 14. There extends from this duct thepipes 17 provided with adjustment valves 18, which thus feed air to eachof the stages of the tower, except the top stage.

At the bottom of the tower, the gas flow which has passed therethroughand has undergone the action of the successive stages of catalyst,leaves by way of the pipe 19. The difiuser-distributor 20 divides thisflow into two, one of which is sent through the pipe 21 into the heatexchanger 12 and the other through the pipe 22 into the. heat recoveryexchanger 23. 4

The outlets 24 and 25 respectively of the heat exchangers 23 and 12 areconnected to form a common duct 26 which directs the cooled gas flowtowards the condensation chambers 27 in which the product obtainedcondenses. The outlet 28 from these condensation chambers is directedtowards the atmosphere after passing through a purifying device of anyform (eg scrubbing tower or cyclone).

The air leaving the compressor 2 is at a temperature of about 80 C. Theexchanger 15 can restore it substantially to atmospheric temperature. Insome cases, as will hereinafter be seen, this exchanger may be a heater.

In the case of the conversion of naphthalene into phthalic anhydride,the heater 7 raises the temperature of the air from 80 to 140 C. Thisheater is formed of parallel tubes 7a immersed in a hot liquid emanatingfrom the heat recovery exchanger 23 and returning thereto after cooling,as indicated by the circuits 30 and 31. I

The naphthalene is injected at this temperature of C. into the chamber 8through the nozzle 9.

The heat exchanger 12 contains a cluster of parallel tubes 12a throughwhich there flow the gases arriving through the pipe 21, while the aircharged with naphthalene is constrained to circulate helically aroundthe said tubes by way of the sheet-metal screw 12b.

This arrangement has the following advantage: The rectilinear tubes 12a,through which the hot gases containing the condensable products pass,can readily be periodically cleaned, while the danger of condensation ofthe impurities of the naphthalene around these hot tubes is negligible.

In the exchanger 12, the mixture of air and naphthalene flowingtherethrough from right to left is brought from 140 C. to about 350 C.,while the gases coming from the pipe 21 and passing through theexchanger 12 from left to right enter the latter at about 380 C. andleave it at about C.

The heat recovery exchanger 23 operates with a liquid flowing over thecluster of rising tubes 23a. The gases entering by way of the pipe 22are also cooled therein from 380 to about 175 C. If it is necessary forthe requirements of the installation, the heat recovery exchanger 23 mayconsist of a number of such exchangers through which the same fiow ofgas passes in series. The temperatures of the liquids employed in thesesuccessive exchangers is thus graded and adapted to each particularapplication.

At the temperature of 175 C., the gases carrying the products of thereaction are introduced into the condensing device 27, which may be ofany type and more especially of the alternating type known as a VonHeyden condenser.

The stages of the tower 14 (FIGURES 2 and 4) consist of beds ofcatalysts in tablet form, for example, in the case of the preparation ofphthalic anhydride, of tabliets of silica gels impregnated with vanadiumpent- 0X1 e.

The said beds 33,, 33 etc., are supported by finemeshed grids 34 34Since the reaction mixture flows from the top downwards in the tower 14,the corresponding pipe 17 (17 17 introduces air through a diffusionnozzle 35 35 below each bed of catalyst. As illustrated, these nozzlesmay be hollow rings formed with peripheral orifices. Disposed below eachnozzle 35 is a grid 36 36 which homogenises the mixture of gas and airin its downward path.

It will be noted that the beds of catalyst are of increasing thicknessand that the pipes 17 to 17 and the nozzles also have increasingdimensions.

Finally, thermometric measuring instruments 37 37 give the temperatureof the gas flow as it enters each stage for the control of theinstallation.

The arrangement illustrated in FIGURES 1 and 2 is suitable in caseswhere pure air is sent into the reaction mixture at each stagesucceeding the first.

On the other hand, when not only air, but also hydrocarbon is sent toeach stage, the installation is advantageously modified as illustratedin FIGURE 3.

All the air supplied by the compressor is sent into the chamber 8, inwhich this air is charged with hydrocarbon in a quantity below theexplosion threshold. At the outlet of the chamber 8, thediffuser-distributor 55 directs the mixture thus obtained on the onehand into the pipe 16 and on the other hand into the exchanger 12. Aswill hereinafter be seen, the quantity of air sent into the pipe 16 isthen much greater than that sent into the exchanger 12. Moreover, theconcentration of the mixture entering the tower, on the one hand,through the pipe 13 and on the other hand through the branches 17 of thepipe 16,

is the same and in addition this concentration is the highest possiblewhich is compatible with the danger of spontaneous ignition, whichaffords a number of advantages, as Will hereinafter be seen.

Each of the stages of the tower is then preferably arranged asillustrated in FIGURE 4.

Eachpipe 17, 17 for example, widens into a chamber 38 in which a smalladditional quantity of fresh hydrocarbon emanating from the general duct4t) can be evaporated by way of the nozzle 41 and homogenised by thegrid 49.

The control of the valve 18 is dependent upon the thermometric detector,for example a thermoelectric couple or a resistance 4,2 varying with thetemperature and sensitive to the temperature at the entrance to thesucceeding bed of catalyst 33 through the electronic amplifier 43 andthe servomotor 44.

The valve 45 for supplying the additional hydrocarbon injected into thechamber 33 is controlled in dependence upon the thermometric detector 46sensitive to the temperature beyond the succeeding bed 33 by Way of theelectronic amplifier 47 and the servomotor 48.

The servomechanism 4-2 43, 44 is adjusted to increase the opening of thevalve 18 in the event of an increase in the temperature of thethermometric device 42 Consequently, if the mixture tends to arrive toohot at the catalysing stage 33 an increase in the quantity of airadmitted through the nozzle 35 cools this mixture, and vice versa if themixture arrives too cold at the level of the bed 33 On the other hand,the valve 45 is normally closed, since the air coming from the pipe 16is already charged with the desired quantity of hydrocarbon.

The servomechanism 46 47, 4-8 is adjusted to permit the opening of thevalve 45 in the event of a reduction in the temperature of thethermometric device 46 Thus, a small addition of oxidisable hydrocarbonenters the mixture, whereby the temperature of the reaction is increasedas it passes through the bed 33 and the temperature at the outlet ofthis catalyst bed is increased.

The formation of a predetermined reaction product from a givenhydrocarbon in the presence of a particular catalyst takes place withits maximum yield at a well defined temperature. Below and above thistemperature, the yield of the reaction rapidly decreases.

Since the passage of the mixture through each catalyst bed is adiabatic,it is therefore desirable that the temperature of this mixture at theentrance to the bed should be lower than the optimum temperature andthat it should leave the bed at a higher temperature.

The optimum temperature difference AT is made the same for all thecatalysing beds, so that the temperature conditions for all of them arethe most favourable. The mixture thus enters the successive beds at thetemperature 0 and leaves them at the temperature 0|AT.

Since the mixture has constant specific heat (substantially equal tothat of the atmospheric air) and the pressure is substantially constant,this temperature difference AT therefore corresponds to a like quantityof oxidised hydrocarbon per unit of volume of this mixture at each ofthe reaction levels.

The gas flows before the stages 33 33 will be called D D D and it willbe assumed that the pipe 17 supplies to any stage a fraction or of theflow at the entrance to this stage.

Under these conditions, in relation to the preceding flow D the flow Dis given by D =D (1-|0L).

The temperature of the air contained in the pipe 16 and fed through thepipes 17 Will be called T In order to restore the temperature of themixture from the temperature 0+AT to the temperature 6, it is necessaryto have aD (0T )=D AT that is to say, the quantity of heat absorbed bythe additional air is equal to the quantity of heat by which the mixturehas become 6 enriched in passing through the corresponding stage.

There is deduced therefrom:

Since AT, 0 and T are constant values, or itself is constant.

In other words, the proportion of air added after each stage isconstant, that is to say, that:

The supplies of mixture must therefore increase with the successivestages with a geometrical progression of ratio 1+.

In the successive gas flows D D the concentration of the hydrocarbon,that is to say, the quantity of hydrocarbon contained per unit ofvolume, decreases but, for the reasons already indicated, the reduction7 of concentration at each of the stages is constant in order to ensurea constant value of AT.

If the hydrocarbon concentration before the stage n in the gas flow isdenoted by C,,, the concentration C' at the outlet from this stage willtherefore be C =C -'y.

In addition, the air fed through the pipe 17 before the stage n maycontain a certain concentration 0 of fresh hydrocarbon (c at the secondstage, a at the third stage, etc.).

By writing that the total quantity of hydrocarbon contained in themixture before the stage n is equal to the quantity emanting from thepreceding stage, plus the quantity introduced by means of the additionalair, there is obtained n-l n 'Y on 1 a By recurrently calculating C fromthe concentration C of the mixture introduced at the inlet to the firststage, there is obtained:

01 C2 1+a 1+a However, there is no reason a priori to introduce at eachstage different concentrations of fresh hydrocarbon into the additionalair and, as is shown by FIGURE 3,

'with the simplest arrangement the same hydrocarbon concentration isgiven to the air sent to all the stages.

We therefore have 0 c =c and by summating the geometrical progressionsdeveloped between the square brackets, we obtain a L 'l- O The Equations1, 2 and 3 make it possible to choose the dimensions of the apparatus inthe various operating conditions. Of all the possible operatingconditions, two afford a particular interest, namely:

(1) The case where all the hydrocarbon entering into reaction isintroducd all at once into the mixture at the bons as starting materialsfor supplying the same final product.

When the hydrocarbon is introduced all at once, the apparatus is thatillustrated in FIGURES 1 and 2, and the term in Equation 3 is zero.

The original concentration C having been determined, the number ofstages of the reaction tower is defined by the condition C Q'y.

In fact, the reaction is substantially complete when the concentrationobtained at a stage falls below the difference of concentration 'yobtained by the oxidation of the hydrocarbon in the passage to each ofthe stages.

In accordance with Equation 3, it is then necessary to have:

the invention:

Example I In the case of naphthalene oxidised by air in the presence ofvanadium oxide, the concentration C at the limit of the explosionthreshold, is about 30 g. per cubic metre of air at the temperature0=350 C.

With the knowledge of the heat evolved by the oxidation of one moleculeof naphthalene into phthalic anhydride, it is possible to choose eitherthe variation of concentration 7, which gives the values of AT andconsequently, in accordance with Equation 1, that of u by choosing thetemperature T of the additional air, or to choose a priori AT in orderto maintain a favourable temperature difference, whereby an and 'y aredetermined, the choice of T remaining arbitrary.

For example, by choosing :3 g. per cubic metre, we find AT=33 C. (thisreaction taking place between 350 and 383 C.).

If the additional air is injected at 20 C., we then find:

Seven stages are sufficient substantially to exhaust the naphthaleneoriginally introduced.

This result, which is surprising because it seems that ten stages atleast would be necessary owing to the fact that the mixture originallycontains 30 g. of naphthalene per cubic metre and the reduction ofconcentration is only 3 g. per cubic metre and per stage, is explainedas follows:

Since additional air is supplied to each stage, the concentration of thehydrocarbon in the mixture decreases more rapidly than if this reductionof concentration were due only to the oxidation of the hydrocarbon. Inthe eatalysing beds succeeding the first one, the same reduction ofconcentration therefore corresponds to the oxidation of a substantiallylarger quantity of hydrocarbon than would be the case if the mixturewere not diluted. On the other hand, the thicknesses of the catalysingbeds must follow a law of rapid increase. Moreover, the quantity of airemployed is important, since the rate of 8 flow D, at the outlet fromthe last bed is in this example given (Formula 2) by D =D (1.1) =1.8D

Therefore, an excess of air is utilised and the rate of flow in the duct16 (FIGURE 1) is almost equal to the flow sent into the duct 5.

The consumption of the compressor set is correspondingly increased.However, since the number of stages is reduced, the grids 34, 36, whichare one of the main causes of pressure drops, are small in number andthe increase in the volume of the circulating gas is compensated for bythe lower pressure necessary for circulating this volume.

Finally, the total balance is favourable because, in addition, almostall the heat generated is recovered in the exchangers 12 and 13.

When additional hydrocarbon is supplied between two successive stages,the equation of the mixture of the flows leaving the stage n can also bewritten:

In this equation, C D and C D represent respectively the quantities ofhydrocarbon contained in the mixture after and before it leaves thestage 11, while (OLC -'Y)D corresponds to the difference between thequantities of hydrocarbon added after this stage and consumed during thepassage through the latter, respectively. Obviously, the mixture mustnot contain more hydrocarbon at the outlet than at the inlet. Therefore,the quantity (10 -7 must be negative or at most zero if it is assumedthat the mixture may contain at the start a certain quantity ofhydrocarbon (and consequently of air), which quantity may be expected tobe found in the mixture at the outlet or may be extracted in a finalstage without injection of fresh hydrocarbon.

Thus, it may be desirable, by making ac -7=0, to supply air andhydrocarbon as the reaction progresses, in quantities such that this airand this hydrocarbon are consumed at each of the stages to which theyare suppiled. In this case, the initial supply of hydrocarbon is usefulonly for maintaining the hydrocarbon concen-' tration at a sufficientlevel for the speed of reaction to remain high, whereby it is possibleto reduce the quantity of catalyst employed and to increase the yield ofthe latter.

In this case, Equations 1 and 2 remain unchanged, Equation 3 beingconsiderably simplified and becoming:

stage is 'y.

We therefore have:

C1 'Y 1 (1+a) 2 In addition, the quantity of catalyst is inverselyproportional to the coefiicien-t K of the reaction.

It will be recalled that a reaction for the oxidation of hydrocarbon inthe presence of anexcess of air is essentially a primary reaction forwhich the speed S of the reaction is governed by the differentialequation:

In this equation x is the number of molecules oxidised after the time t,and a is the number of hydrocarbon molecules intially present.

Finally, the volume V of catalyst necessary at stage n may be written:

1 a 211-2 V11 2C1 Y(l l a) However, 7 is only a small fraction of C anda is also a small fraction in relation to unity. Consequently, if Itremains relatively small, it. is possible to neglect the quantity'y(l+oc) and to write It will thus be seen that the quantity of catalystemployed in the successive beds are a geometrical progression whoseratio is (1+Oc) As previously, the number of stages to be used isobtained by assuming that the concentration after the nth stage becomeslowert-han 7, that is to say, in accordance with Equation 3;;

In the case where, for the production of phthalic anhydride, o-xylene isemployed, which is oxidised at a temperature =350 C. using vanadiumoxide catalyst, the concentration of the hydrocarbon in the mixture isalways advantageously made as high as possible, provided that it remainsbelow the explosion threshold. In fact, the higher the concentration thehigher the speed of reaction and the larger the quantity of catalystreduced.

Thus, as in Example I, there may be chosen As in Example I, with theknowledge of the heat evolved by the oxidation of one molecule ofo-xylene into phthalic anhydride, it is possible to chose 7 or AT and tocalculate the other from the chosen quantity.

Thus, if 1 :3 g. per m. is chosen, then AT=23 C.

In this case, a is determined by the relation ocC 'y=0 or 30ot3=0.

Therefore, a must be equal to one-tenth (0.1).

Equation 1 then gives the value of T 10 3E & 19ers 1og1.l log 1.1

The number of stages required is therefore 27. This result can also bereached intuitively.

A certain quantity of hydrocarbon mixed with airis introduced in thefirst stage of the apparatus. 'In the succeeding stages, air andhydrocarbon are simultaneously added in quantities such that the airadded is heated to the lower temperature level of the reaction, whilethe mixture is brought from the upper temperature of the reaction to thesaid lower temperature. In addition, the quantity of hydrocarbon addedcorresponds to that which is consumed at each stage. The initialquantity of hydrocarbon introduced is therefore maintained and isprogressively diluted in an increasing quantity of air.

The reaction is therefore completed when the concentration of theinitial quantity of hydrocarbon falls below the drop in concentration 7at each stage.

In the numerical example just considered, the initial concentration is30 g. .per cubic metre of air and the variation of concentration at eachstage is 3 g. per cubic metre. The concentration of hydrocarbon willtherefore fall below 3 g. per cubic metre when the volume of mixture,i.e. substantially the volume of air, has beenmultiplied by ten. Now,this volume of air increases in accordance with the law of flow given bythe Equation 2:

The reaction will be complete When i.e. as before log 10 log 10 log(ll-a) log 1.1

This calculation also shows that the rate of flow D is only one-tenth ofthe final rate of flow.

Moreover, both before the first stage and in the course of thesucceeding stages, the air employed contains a constant concentration (Cc of hydrocarbon.

This has two important consequences:

(I) No excess of air over that necessary for maintaining the hydrocarbonconcentration below the explosion threshold is ever introduced into themixture, and

(2) In the first stages of the reaction apparatus, the quantity of gascirculating is very small and the gas flow reaches its completemagnitude only towards the last stages, notably at the last stage.

Consequently, the work which must be done by the compressor is a minimumsince, on the one hand, the quantity of circulating air is reduced towhat is essential and, on the other hand, the pressure drops due to thepassage through the successive catalysing beds are also reduced to thosewhich are absolutely necessary for causing the reaction mixture usefullyto pass through the said catalysing beds.

Such an arrangement affords a further advantage.

It is known that, starting with different hydrocarbons, a particularcatalyst is substantially specific to the preparation of a givenoxidised substance. Thus, the higher oxides of vanadium makes itpossible to prepare phthalic anhydride both from naphthalene and fromxylenes, and notably from o-xylene. Only the speed of reaction K isdifferent.

In accordance with Equation 4, the volumes of catalysts increase in ageometrical progression whose ratio is (1+a) and in additionproportionally to the rate of flow D and, finally, inverselyproportionally to K and 1:0 C1.

Assuming that the concentration (C the rate of flow before the first bedof catalyst (D and the quantity (or) are constant, the volume ofcatalyst depends only upon K.

For example, in the case of o-xylene, the speed of reaction K is threetimes higher than in the case of naphthalene.

Therefore, a quantity of catalyst one-third as large will be sufficientto ensure the reaction in the first stage. Thereafter, in both cases,the laws of increase of the quantities of catalysts are the same.

' It is thus possible, as shown in FIGURE 5, to conl1 struct thereaction tower with a succession 'of numerous catalysing beds 33 to 33for which the quantity of catalyst increases in accordance with theindicated geometrical progression from the first (at the top) to thelast (at the bottom).

The pipe 16 with its branches 17, and the pipes 40 for the additionalsupply of hydrocarbon extend over the entire height of the tower. Inaddition, in the top portion of the latter, the pipe '13 (FIGURE 1) isextended by an admission collector comprising the branches 51-providedwith valves opening between two beds of the upper stages. Similarly, adischarge pipe 52 provided with branches comprising valves 53 leads tothe discharge pipe 19.

Under otherwise equal conditions, for the treatment of o-xylene thereaction may commence at the bed 33 and end at the bed 33,, (portion Iof the tower), the gases entering through the branch 51 being dischargedafter reaction through the branch 53,,.

On the other hand, when naphthalene is treated, the reaction will startat the bed 33,, (in which the quantity of catalyst is, by geometricalprogression, equal to three times the quantity contained in the bed 33the air containing naphthalene entering at 51 and will continue to thebed 33 (portion II of the tower).

In both cases, the same adjustments of the admission of additional airand hydrocarbon are suitable, since the concentrations C and C areadvantageously maintained at the value corresponding to the explosionthreshold.

The same installation (depending upon the hydrocarbon avail-able asstarting material) is suitable in both cases for the same totalproduction.

If the production is modified by varying the rate of flow D it is alsopossible to choose the discharge stage and the admission stage in suchmanner that the quantities of catalysts of the beds employed are alsosuitable. It is thus possible to change the rate of production of theinstallation without its even being necessary to stop the latter.

It is likewise possible with a suitable catalyst, for example forobtaining phthalic anhydride, to inject into the How of air leaving thecompressor before the exchanger 7 (temperature about 80 C.), o-xyleneand, after this exchanger (temperature 140 C.), naphthalene in ordersimultaneously to oxidise these two gases.

The tower may be surrounded by a framework by which it can be raised andthe various stages may consist of cylindrical rings which are detachablefor the periodic renewal of the catalyst or for cooling the latter.

I claim:

1. A method of continuously oxidizing a hydrocarbon comprising formingand advancing a mixture of hydrocarbon with a large excess ofatmospheric air; bringing the mixture in successive individuallyadiabatic stages into contact with successive and increasing quantitiesof catalyst at a temperature 0 just below the optimum reactiontemperature; limiting the reaction in each stage to a predeterminedtemperature rise AT; and cooling the mixture after said contact from thetemperature 0+AT above the optimum reaction temperature to the lowertemperature 0, the said cooling being effected by addition ofatmospheric air at a constant temperature T in an amount for each stageequal to a fraction tity of catalyst of the following stage, saidsuccessive increasing quantities of catalyst being proportional to ageometrical progression of ratio (l+a) 3. A method of continuouslyoxidizing a hydrocarbon comprising forming and advancing a mixture ofhydrocarbon with a large excess of atmospheric air, bringing the mixturein successive individually adiabatic stages into contact with successiveand increasing quantities of catalyst at a temperature below the optimumreaction temperature, and cooling the mixture, after the said contact,from the temperature, above the said optimum temperature, which itreaches, to the said lower temperature, the said cooling being effectedby the addition of air at a constant definite temperature, thetemperature rise of the mixture between the beginning and the end of thecontact with each quantity of catalyst being constant for each stage andthe amounts of additional air being increased in a manner that thesupplies of mixture increases in a geometrical progression withsuccessive stages.

4. A method of continuously oxidizing a hydrocarbon comprising formingand advancing a mixture of hydrocarbon with a large excess ofatmospheric air; bringing the mixture in successive individuallyadiabatic stages into contact with successive and'increasing quantitiesof catalyst at a temperature below the optimum reaction temperature, andcooling the mixture, after the said contact, from the temperature, abovethe said optimum tempera ture, which it reaches, to the'said lowertemperature limiting the reaction in each stage to a predeterminedtemperature rise, the said cooling being effected by the addition of anappropriate quantity of air at a constant definite temperature admixedwith a quantity of fresh hydrocarbon equal to that which is consumed bythe contact with the next succeeding quantity of catalyst while thesupplies of mixture to the successive stages is increased in ageometrical progression.

5. A method of continuously oxidizing a hydrocarbon comprising formingand advancing a mixture of hydrocarbon with a large excess ofatmospheric air; bringing the mixture in successive individuallyadiabatic stages into contact with successive and increasing quantitiesof catalyst at a temperature below the optimum reaction temperature, andcooling the mixture, after the said contact, from the temperature, abovethe said optimum temperature, which it reaches, to the said lowertemperature, the

said cooling being effected by the addition of increasing quantities ofair at a constant definite temperature providing supplies of mixtureincreasing according to a geometrical progression in the successivestages and the quan tities of catalyst being great-er in the saidsuccessive stages in a geometrical progression in a ratio equal to thesquare of the progression ratio of the rates of mixture supplies.

6. An apparatus for continuous catalytic partial oxidation ofhydrocarbon with air wherein the desired oxidation reaction takes placewithin a narrow temperature range including a vertical tower having aplurality of horizontally supported catalyst beds, the quantity ofcatalyst in the beds increasing progressively from top to bottom of thetower, said beds being spaced apart to provide a substantial free spaceabove each bed; means in the free space above the uppermost catalyst bedfor evenly diffusing a mixture of air and hydrocarbon into said space;means for maintaining the entering charge of air and hydrocarbon at apredetermined constant temperature below the desired reactiontemperature; separate, individual means in the free spaces above each ofthe catalyst beds below the first, for diffusing air into the respectivefree spaces; corresponding separate means for separately regulating theamount of air supplied to the diifusing means, individual temperaturesensing devices in each of the free spaces except the first; meansconnecting the respective temperature sensing devices with the respec'tive air regulating means whereby a sufficient amount of air isintroduced to cool the reaction mixture, which has been heated bypassage through the catalyst bed next above the free space, tosubstantially the temperature of the charge entering the tower; andmeans for conducting away the reaction product emerging from thelowermost catalyst bed.

7. The apparatus according to claim 6 including means for addingadditional feed hydrocarbon to the air supplied to the free spacesbetween the catalyst beds.

8. The apparatus according to claim 7 wherein there is a common sourcefor all of the air supplied to the free spaces including means forsupplying feed hydrocarbon to said source and means for individuallysupplying additional amounts of hydrocarbon to each of the difiusingmeans.

9. The apparatus according to claim 6 wherein the ratio of increase ofcatalyst in the beds in the tower is substantially a geometric ratio.

References Cited by the Examiner UNITED STATES PATENTS Peter's 23288Cummings 23-488 Mathy 23288 Gilmore 23288 Krantz 260346.8

Korin 23-488 Benichou et al. 260-346.4 Benichou et a1. 260346.4

HENRY R. JILES, Acting Primary Examiner.

15 IRVING MARCUS, Examiner.

1. A METHOD OF CONTINUOUSLY OXIDIZING A HYDROCARBON COMPRISING FORMINGAND ADVANCING A MIXTURE OF HYDROCARBON WITH A LARGE EXCESS OFATMOSPHERIC AIR, BRINGING THE MIXTURE IN SUCCESSIVE INDIVIDUALLYADIABATIC STAGES INTO CONTACT WITH SUCCESSIVE AND INCREASING QUANTITIESOF CATALYST AT A TEMPERATURE B JUST BELOW THE OPTIMUM REACTIONTEMPERATURE; LIMITING THE REACTION IN EACH STAGE TO A PREDETERMINEDTEMPERATURE RISE $T; AND COOLING THE MIXTURE AFTER SAID CONTACT FROM THETEMPERATURE B+$T ABOVE THE OPTIMUM REACTION TEMPERATURE TO THE LOWERTEMPERATURE $, THE SAID COOLING BEING EFFECTED BY ADDITION OFATMOSPHERIC AIR AT A CONSTANT TEMPERATURE T0 IN AN AMOUNT FOR EACH STAGEEQUAL TO A FRACTION A=$T/(B-T0) OF THE REACTION MIXTURE FLOW WHEREBY THESUPPLIES OF MIXTURE TO THE SUCCESSIVE STAGES INCREASES IN A GEOMETRICALPROGESSION OF RATIO I+A WITH SUCCESSIVE STAGES.
 6. AN APPARATUS FORCONTINUOUS CATALYTIC PARTIAL OXIDATION OF HYDROCARBON WITH AIR WEHEREINTHE DESIRED OXIDATION REACTION TAKES PLACE WITHIN A NARROW TEMPERATURERANGE INCLUDING A VERTICAL TOWER HAVING A PLURALITY OF HORIZONTALLYSUPPORTED CATALYST BEDS, THE QUANTITY OF CATALYST IN THE BEDS INCREASINGPROGRESSIVELY FROM TOP TO BOTTOM OF THE TOWER, SAID BEDS BEING SPACEDAPART TO PROVIDE A SUBSTANTIAL FREE SPACE ABOVE EACH BED; MEANS IN THEFREE SPACE ABOVE THE UPPERMOST CATALYST BED FOR EVELY DIFFUSING AMIXTURE OF AIR AND HYDROCARBON INTO SAID SPACE; MEANS FOR MAINTAININGTHE ENTERING CHARGE OF AIR AND HYDROCARBON AT A PREDETERMINED CONSTANTTEMPERATURE BELOW THE DESIRED REACTION TEMPERATURE; SEPARATE, INDIVIDUALMEANS IN THE FREE SPACES ABOVE EACH OF THE CATALYST BEDS BELOW THEFIRST, FOR DIFFUSING AIR INTO THE RESPECTIVE FREE SPACES; CORRESPONDINGSEPARATE MEANS FOR SEPARATELY REGULATING THE AMOUNT OF AIR SUPPLIED TOTHE DIFFUSING MEANS, INDIVIDUAL TGEMPERATURE SENSING DEVICES IN EACH OFTHE FREE SPACES EXCEPT THE FIRST; MEANS CONNECTING THE RESPECTIVETEMPERATURE SENSING DEVICES WITH THE RESPECTIVE AIR REGULATING MEANSWHEREBY A SUFFICIENT AMOUNT OF AIR IS INTRODUCED TO COOL THE REACTIONMIXTURE, WHICH HAS BEEN HEATED BY PASSAGE THROUGH THE CATALYST BED NEXTABOVE THE FREE SPACE, TO SUBSTANTIALLY THE TEMPERATURE OF THE CHARGEENTERING THE TOWER; AND MEANS FOR CONDUCTING AWAY THE REACTION PRODUCTEMERGING FROM THE LOWERMOST CATALYST BED.